I don’t think they need any reason for an extension the way things are at the moment but going by what I’ve heard other say here that having made a large AP on your current car three years ago is a good enough reason and 18,000 miles is not much far out anyway.
I’ve extended by a year without even talking to anyone using the form below but had just under 15,000 due to lockdowns.
https://www.motability.co.uk/contact/request-a-lease-extension/
Just to add you can’t put the form in until 3 months before the end of lease, in your case next Sunday / Monday will be fine